1. Technical Field
This invention relates to a sealed bellows assembly and more particularly to such a sealed bellows assembly useful in an aircraft pneumatic bleed pressure controller for regulating pneumatic pressure relative to aircraft altitude.
2. Background Information
A sealed bellows assembly is useable when differential pressure is translated in to a function of mechanical motion via differential length of the bellows assembly. Such pressure sensitive mechanisms are utilized in aircraft as a means of sensing or controlling systems relative to aircraft altitude. Differential pressure between a reference pressure inside the bellows assembly, usually but not necessarily O PSIA, and the external pressure produces a function corresponding to the change in mechanical length of the bellows assembly. Numerous differential pressure versus mechanical properties functions of the bellows assembly are possible.
Previous designs are dependent upon the use of a dampening fluids, plastic, or other dampening materials to ensure that the bellows assembly does not succumb to vibration inherent in aircraft and engines, and performs as required in the rugged environments found on aircraft and engines. Typically, aircraft applications are extremely demanding in that there can be significant levels of vibration and temperature present along with damaging chemicals, ranging from fuels and oils to chemical cleaners.
Design limitations with other bellows assemblies cause operational problems. The primary problem is the inability to evacuate completely the bellows assembly for precise reference pressure due to the vapor pressure of the dampening fluid or fluids and the carbonaceous guide assemblies. This causes the reference altitude versus ambient pressure function to be inaccurate with time while the struggle for chemical equilibrium within the sealed bellows assembly causes the reference pressure to change until the mechanical or operational limits of the bellows have been reached. The resulting internal pressure changes cause system inaccuracies, and incorrect system operation. At periodic intervals, the bellows assembly reference pressure function shifts past the point to which the controller it is mounted to can be compensated, and the bellows assembly requires replacement.
Variable reference pressure shifts of prior bellows assemblies also causes technical problems. The primary problem is the controller assembly uses a changing reference, which causes system imperfection and deteriorating system accuracy. Secondary problems with the incorrect reference settings occur in unit to unit variance which results in engine or other systems functions being performed without required accuracy and precision. The changes in the bellows and controller precision are not necessarily consistent with time, but vary from unit to unit based on individual bellows"" construction, time since built, temperature, environment, and running time on aircraft or engines.
Thus a need exists for a vibrational optimized, evacuated bellows assembly.
Briefly, the present invention satisfies the need for a sealed bellows assembly that will reproduce the original pressure settings for extended periods of time.
The present invention provides an evacuated and hermetically sealed structure. Mechanical attachment means are secured to the assembly at opposite ends thereof. The bellows assembly is vibrationally optimized to a predetermined frequency. The bellows assembly frequency is tuned so that it is not in the frequency range of its operating environment.
These and other features and advantages of this invention will become apparent from the following detailed description of the various aspects of the invention taken in conjunction with the accompanying drawing.